Add gitlab-opencode to GitLab docs

The current GitLab page describes OpenCode integration through GitLab Duo.

GitLab Duo is a paying functionality and is limited to workflows supported by GitLab.

GitLab-OpenCode is a community project that offers more flexiblity, better customization and easier setup to use OpenCode in GitLab. On the downside, it does not have the level of integration into GitLab as Duo does.